1. Field of the Invention
The present invention relates to a method of measuring length with a scanning type electron microscope (hereinbelow, called as SEM), and in particular, relates to a method of measuring length with an SEM which is suitable for length measurement and detection for a semiconductor device.
2. Conventional Art
Recent SEMs, in particular, SEMs for length measurement are required that a measured length value shows a true value and does not includes a variation and offset depending on such as constitution and film type of a specimen to which length measurement has been performed. Conventionally, as a size calibration measure for the length measurement SEM a pitch pattern is prepared by making use of a material such as Al and W which generates less charges on a Si, and through matching the measured length value to the prepared pitch pattern a length measurement calibration value for the length measurement SEM is obtained.
Further, for recent length measurement SEMs, in order to enhance secondary electron resolution a retarding electric field method (hereinbelow, simply called as a retarding method) is employed in which an acceleration voltage of incident electrons is increased and a negative voltage is applied to the side of a specimen to retard the acceleration voltage of the incident electrons. More specifically, in comparison with a conventional method wherein zero voltage is applied to the side of a specimen, the retarding method is constituted in such a manner that an acceleration voltage for electrons emitted from an electron gun unit is raised by +Vr and a voltage xe2x88x92Vr is applied to the specimen, thereby, electrons discharged from the electron gun unit with acceleration voltage of V+Vr pass an objective lens, thereafter, are retarded by the voltage xe2x88x92Vr applied to the side of the specimen and finally make incident to the specimen with an acceleration voltage V. As features of the retarding method, the followings are enumerated, in that since the incident electrons at the primary side pass below to the objective lens with the acceleration voltage V+Vr, a chromatic aberration of lenses can be reduced, and since secondary electrons generated from the specimen are accelerated toward a detector by the electric field due to xe2x88x92Vr, a collection efficiency of the secondary electrons is enhanced.
However, in the SEM employing the above explained retarding method, since the acceleration voltage (hereinafter, sometimes called as a landing voltage) of the electron beams incident onto the specimen varies depending on the voltage applied to the side of the specimen, there arises a problem that in case when a specimen to be observed is covered by an insulation film or an insulation film is deposited on the surface thereof the landing voltage varies and the magnification (or field of view) can deviate. Since the size calibration for the length measurement SEM is performed under a constant landing voltage and a constant magnification, therefore, if one of the two varies, there causes a problem that the measured size value varies. Namely, when the landing voltage varies, a size error is caused and there arised a problem that the measured size value is not a true value.
The above problem was likely caused in the conventional length measurement SEM which does not employ the retarding method. Namely, there arose the problem likely that the landing voltage thereof varies because of a variation in surface potential of a specimen, as a result a size error is caused, and therefore, the measured size is not a true value. Table 1 below shows results of pitch length measurement in a Si pattern formed on a Si wafer, a pattern on B-PSG film having film thickness 400 nm, a pattern on SiO2 film having film thickness 10 nm, a pattern on SiO2 film having film thickness 100 nm, a pattern on SiN film, a pattern on HTO+SiN film (35 nm) and a pattern on SiON film having film thickness 25 nm formed on a Si wafer, and a pattern on actual devices A and B.
Design values of length measured pitch are 350 nm, 400 nm, 500 nm, 600 nm, 700 nm and 800 nm. The unit of all numeral values in the Table is nm.
FIG. 18 illustrates a relationship between pitch design values (in abscissa) and SEM length measurement values (in ordinate) obtained from data in connection with Si pattern, pattern on B-PSG film and pattern of actual device A among the length measurement data as shown in Table 1. It is observed that although the pitch length measurement values of the Si pattern substantially coincide with the designed values, the pitch length measurement values of the patterns on the B-PGS insulation film and the actual device A greatly deviate from the respective designed values. It is considered that such errors in the measured values are caused due to the change-up of the specimen surface irradiated by electron beams during the pattern length measurement on the B-PSG insulation film and the actual device A.
The present invention is achieved in view of the problem encountered during the length measurement with the conventional SEMs, and an object of the present invention is to provide a method of measuring length with an SEM which prevents a conventional length measurement error depending on such as specimen constitutions and film types.
A method of measuring length with an SEM according to the present invention which resolves the above problem, is characterized in that the method comprises the steps of: performing length measurement with the SEM an already know pattern provided in advance in a predetermined region on a specimen; obtaining a magnification correction coefficient through comparison of the length measurement result with the designed value of the already known pattern; and determining a true size by multiplying a measured length value of a measurement point performed by the SEM by the obtained magnification correction coefficient.
In the above method, a specimen is used in which a particular test pattern such as lines and spaces and contact holes for obtaining correction data is in advance provided, for example, in a scribe in a semiconductor wafer specimen and in a free space in a chip. Then, the length measurement of the test pattern is performed and the magnification correction coefficient for correcting a length measurement error is determined, thereby, a size measurement error by the SEM, which is caused due to the charge up of the specimen surface (in that the surface potential variation of the place where incident electrons are irradiated), is compensated.
Further, a method of measuring length with an SEM according to the present invention, is characterized in that the method comprises the steps of: performing length measurement with the SEM a pattern on a specimen; obtaining a magnification correction coefficient through comparison of the length measurement. result with the designed value of the pattern; and determining a true size by multiplying a measured length value of a measurement point performed by the SEM by the obtained magnification correction coefficient.
In the above method, without providing on the specimen a test pattern used exclusively for determining the magnification correction coefficient and by making use of a pattern which permits measurement of a line width, a space width or a pitch among patterns in an actual device, the magnification correction coefficient is determined which compensates errors due to the charge-up of the specimen surface.
The magnification correction coefficient can be obtained for every specimen. When the specimen is a semiconductor wafer, the magnification correction coefficient can be obtained for every chip containing a measurement point. Further, the magnification correction coefficient can be obtained by making use of a pattern near a measurement point. The charge-up condition of the specimen surface delicately varies depending on a variety of causes such as structure of specimen, material thereof and a holding state of the specimen such as by a stage. Accordingly, in order to accurately compensate an error in a length measurement with the SEM caused by the charge-up, it is preferable to obtain the magnification correction coefficient by making use of a pattern in a position close to a measurement point as much as possible, because the charge-up condition of the position is considered close to that in the measurement point. When obtaining the magnification correction coefficient for every chip, or when obtaining the magnification correction coefficient by making use of a pattern near a length measurement point, the magnification correction coefficient is usually determined by making use of a pattern in an actual device without using a test pattern exclusively provided for determining the magnification correction coefficient.
When obtaining the magnification correction coefficient, it is preferable to compare a pitch length measurement result in a pattern with a designed value. This is because such as a line width and a space width in a pattern formed by exposing light may deviate from the designed value due to such as proximity effect and light interference, however, the pitch thereof mostly assumes the designed value. When obtaining pitch length measurement result, it is preferable to average a plurality of pitch length measurement results. As an embodiment for length measurement when the pitch length measurement result is used for obtaining the magnification correction coefficient, there is a method in which a pitch together with a line width and a space width at a length measurement point within a view field of the SEM are collectively measured and the measured length values of the line and space are corrected by the magnification correction coefficient obtained from the pitch length measurement result.
Further, a method of measuring length with an SEM according to the present invention, is characterized in that the method comprises the steps of: causing the SEM auto-focusing on a measurement point by making use of an output of a sensor which detects distance from an objective lens to a specimen; detecting an exciting current xcex94Iobj of the objective lens representing an outof focus by monitoring a variation of a specimen signal being emitted from the specimen while varying the exciting current of the objective lens; converting the exciting current xcex94Iobj of the objective lens into an acceleration voltage xcex94V for incident electron beams onto the specimen; changing the acceleration voltage for the incident electron beams onto the specimen by xcex94V; and performing the length measurement of the measurement point.
The auto-focusing can be performed by controlling the exciting current for the objective lens as well as by mechanically moving such as a specimen stage which carries the specimen. With this method, a variation in electron beam acceleration voltage caused due to the charge-up of the specimen surface is canceled and the acceleration voltage for the electron beams incident onto the specimen can be kept at a set value, thereby, an error in the SEM length measurement value due to the charge-up of the specimen surface is eliminated.
Further, a method of measuring length with an SEM according to the present invention, is characterized in that the method comprises the steps of: causing the SEM auto-focusing on a measurement point by making use of an output of a sensor which detects distance from an objective lens to a specimen; detecting an applied voltage xcex94Vb on the specimen representing an outof focus by monitoring variation of a specimen signal being emitted from the specimen while varying the voltage applied on the specimen; changing the acceleration voltage for the incident electron beams onto the specimen by xcex94Vb; and performing the length measurement of the measurement point.
The auto-focusing can be performed by controlling the exciting current for the objective lens as well as by mechanically moving such as a specimen stage which carries the specimen. With this method, like the previous method, a variation in electron beam acceleration voltage caused due to the charge-up of the specimen surface is canceled and the acceleration voltage for the electron beams incident onto the specimen can be kept at a set value, thereby, an error in the SEM length measurement value due to the charge-up of the specimen surface is eliminated.
Further, a method of measuring length with an SEM according to the present invention, is characterized in that the method comprises the steps of: determining in advance a correspondence between acceleration voltage of electron beams incident onto a specimen and magnification correction coefficient; determining an actual acceleration voltage of the electron beams incident onto the specimen at a length measurement point; determining a magnification correction coefficient corresponding to the actual acceleration voltage of the electron beams incident onto the length measurement point with reference to the correspondence determined in advance; performing a length measurement at the measurement point; and determining a true size by multiplying the measured length value by the magnification correction coefficient.
In the above method, any steps which determine the actual acceleration voltage for the electron beams incident onto the length measurement point on the specimen can be used, however, such step can, for example, include a step of determining an objective lens correction current xcex94Iobj for eliminating an outof focus under the condition of objective lens excitation corresponding to the acceleration voltage for the electron beams set at the SEM and a step of converting the objective lens correction current xcex94Iobj into an acceleration voltage for electron beams incident onto the specimen. Alternatively, the step can be realized by determining a specimen application voltage xcex94Vb for eliminating an outof focus under the condition of objective lens excitation corresponding to the acceleration voltage for the electron beams set at the SEM.
According to the length measurement method of the present invention which uses an SEM, a true size value can be obtained by compensating a length measurement error due to potential variation at the specimen surface.
Thereby, even with the commonly used length measurement SEM which does not employ the retarding method, the size measurement error due to the charge-up of the specimen surface caused by the incident electron irradiation is sufficiently corrected and a true value can be obtained regardless to the constitutions of the specimen and the types thereof. Namely, when the specimen is charged up, the surface potential of the specimen varies, and in association therewith a magnification deviation due to an error in landing voltage for incidence beams is caused which implies inclusion of an offset value into the length measurement value, however, such problem is resolved with the present invention, therefore, even if a variation in surface potential of the specimen due to electron beam irradiation is caused, a true length value can always be measured.
Further, with the length measurement SEM which uses the retarding method, the conventional problem of the deviation in magnification due to a setting error of the landing voltage is resolved. Namely, the problem is resolved which causes a size error depending on specimens to be subjected to length measurement such as when an insulation film is deposited on the surface of the specimen or when an interlayer structure for the portion where the electron beams are irradiated varies, and a true length value can always be measured.
Further, according to an embodiment of the present invention, the acceleration voltage for electron beams incident onto the specimen can always set at the design value regardless to the constitution of the specimen and the type thereof.